This invention relates to an ultrasonic flow-measuring method serving to measure the flow rate of a medium traveling through a measuring conduit, by means of two ultrasound transducers which in the flow direction of the medium are offset relative to each other and both of which alternate in emitting ultrasonic pulses, while the respective other ultrasound transducer receives the emitted ultrasonic pulses and the flow rate is determined as a function of the run times of the ultrasonic pulses received by the respective other ultrasound transducer.
As stated above, the ultrasonic flow-measuring method serves to measure the flow rate of the medium traveling through the measuring conduit. In that context the following should be noted: the medium traveling through the measuring conduit typically has a particular flow pattern. For example, if the flow is laminar, that may be a pattern in which the radial gradient of the speed progression is linear. In fact, at different points within the cross section of the measuring conduit the medium usually travels at different speeds, meaning that the flow rate measured will in all cases be a rate that is averaged along the measuring path, i.e. the connecting line between the two ultrasound transducers.
Moreover, this averaged flow rate does not necessarily represent the mean flow rate of the entire medium within the measuring conduit, but depends on the vector of the measuring path through the medium. The mean value of the flow rate of the entire medium traveling through the measuring conduit can be determined by a measurement employing two ultrasound transducers for as long as the measuring conduit is a rotationally symmetric pipe, the flow pattern as well is rotationally symmetric and the connecting line between the two ultrasound transducers intersects the longitudinal axis of the measuring conduit.
With regard to the measuring conduit it should be noted that typically it is in fact rotationally symmetrical, meaning that is has a circular cross section. In theory, however, the measuring conduit may have any cross-sectional form. In addition, the measuring conduit may be a closed tube or it may be an open trough.
The fundamental measuring principle employed in the above-described ultrasonic flow-measuring method is based on the fact that, in the direction of the flow, the ultrasound pulses travel through the moving medium with one speed component, and in the opposite direction, they travel with another speed or rate component, so that the resulting run times of these ultrasonic pulses will be as follows:                               T          ab                =                  L                      c            +                                          v                m                            ⁢              cos              ⁢                                                           ⁢              θ                                                          (        1        )                                                      T            ba                    =                      L                          c              -                                                v                  m                                ⁢                cos                ⁢                                                                   ⁢                θ                                                    ,                            (        2        )            where Tab is the run time of an ultrasonic pulse with a rate component in the flow direction, Tba is the run time of an ultrasonic pulse with a rate component in the opposite direction of the flow, L is the distance between the two ultrasound transducers, c is the acoustic velocity in the medium flowing through the measuring conduit, vm is the average flow rate of the medium along the measuring path i.e. the connecting line between the ultrasound transducers, and θ is the angle between the connecting line of the two ultrasound transducers and the direction of flow.
Using the equations 1 and 2, the average flow rate of the medium can be calculated as follows:                               v          m                =                              L                                          2                ·                cos                            ⁢                                                           ⁢              θ                                ⁢                                    ⌊                                                                    T                    ba                                    -                                      T                    ab                                                                                        T                    ba                                    ·                                      T                    ab                                                              ⌋                        .                                              (        3        )            
It is evident from equation 3 that the acoustic velocity in the medium does not constitute a function in calculating the flow rate of the medium, meaning that according to this calculation, the flow rate of the medium is unrelated to the acoustic velocity in the medium.